Complex electronic devices, such as two-way radios, cellular telephones, and computers are increasingly becoming portable as electronics are integrated into smaller, more efficient packages. Concurrently, there has been an increase in demand for quality battery systems to power these portable devices. For devices which have a high-power demand, such as cellular phones and portable computers, rechargeable battery systems, in particular nickel cadmium and nickel metal hydride-based systems are the most economic choice.
The use of batteries has presented designers of such devices with an interesting challenge; without the benefit of a regulated power supply, the device must operate over an input voltage range rather than at one particular voltage. The battery voltage is continuously changing as different device subsystems turn on and off. The problem then is to get the device to operate consistently while the bias voltage of the electronics is continuously changing. Similarly, varying applications and demands upon the device may require the battery system to have a particular current output which varies over time and in response to the particular device.
The means exist to regulate battery voltage and/or current, either in the device or in the battery itself. For example, integrated linear voltage regulators that transform raw battery voltage into a stable voltage for the device are available. Linear regulators, however, in light of the fact that operation time is a critical market feature, are prohibitively inefficient for all but very low current subsystems of the device. Switch mode regulators can provide a more efficient means of regulating battery voltage but cost and complexity also prohibit the use of such means to low current subsystems. Therefore, design of portable electronic devices is left to contend with a battery voltage that may change as much as 100% from a fully discharged state to a peak voltage while being charged.
Similarly, the current drawn from the battery may also vary depending upon the impedence. The impedence is a function of the particular circuit elements employed in the circuit, and the number of such elements actually functioning at a given time. Existing battery systems do not have the ability to reconfigure themselves to address changing current (or voltage) demands.
Accordingly, there exists a need to provide a battery pack capable of providing a variable voltage or variable current output. Further, such a battery pack should be fabricated so as to provide a large current and/or voltage while minimizing the amount of space needed to provide such a device.